The present invention relates generally to color management in graphic arts (CA) publishing, and more specifically to techniques for rendering specified colors.
Reproduction of images in today's publishing industry almost always involves their intermediate storage in electronic form. Numerical values encode the colors in such images. Publishers typically deliver encoded images and text to printing contractors, who use production grade printing devices to produce books, magazines, or other printed materials. The central problem for color management is to make published images look the way the designer intended.
Current practice includes many techniques that address this problem. For GA publishing to meet designers' expectations about color, designers must first express those expectations. While they may specify colors using a standard color identification scheme such as the CIE XYZ system or the Pantone system, it is just as likely that they will point to an existing object (for example, a photograph) or a video display and say "Make it look like that."
Given that what counts is appearance, numerical encoding schemes should facilitate, rather than hinder, the process of obtaining printed output that "looks right." Unfortunately, the multiplicity of encodings is a source of problems. For more than a century scientists have known that human eyes have three kinds of receptors. Spectral response curves characterize these kinds of receptors, but they can loosely be said to respond to the three basic colors, red, green, and blue. Humans perceive the color of light arriving at the eye on the basis of the different responses to that light by the three kinds of receptors.
FIG. 1 is a simplified view showing how different colors can be obtained by combinations of three primaries. The drawings are simplified in the sense that a gradation of colors between a given pair of primaries can be obtained by different combinations of the adjacent primaries. The lefthand portion of FIG. 1 shows the case of additive primaries, red (R), green (G), and blue (B). In principle, the appearance of any color can be simulated by starting with black (no light), and adding various proportions of red, green, and blue light (R, G, and B). When the amounts of R, G, and B are equal and at the maximum intensity, the result is white light. As can be seen, cyan can be obtained by combining G and B, magenta by combining R and B, and yellow by combining R and G.
One advantage of RGB is that it represents one model of human vision, and can form the basis for designing input devices (such as scanners, colorimeters, and digital cameras) that imitate the eye, and output devices (such as monitors) that fool the viewer into believing that many colors are being seen. A computer monitor, for example, simulates colors by exciting red, green, and blue phosphors to emit at different intensities. A scanner imitates color vision by measuring the intensities of red, green, and blue light reflected from a piece of artwork or a photograph or transmitted through a slide.
The righthand portion of FIG. 1 shows the case of subtractive primaries. In principle, any color can be simulated by starting with white light, and removing selected amounts of red, green, and blue light. This forms the basis of color printing, where a portion of white light reflecting from a printed page is removed by filtering pigments, commonly known as inks or dyes. Red is removed by a cyan (C) ink, green by a magenta (M) ink, and blue by a yellow (Y) ink. C, M, and Y are sometimes referred to as the primary colorants. As can be seen, green can be obtained by a combination of C and Y, red by a combination of M and Y, and blue by a combination of C and M. These combinations are achieved by covering the area with different relative amounts of the primary colorants. Colorant amounts (or colorant values) are often expressed in dot percentages.
In principle, some combination of C, M, and Y should give a neutral color (gray). In most dye sets, a neutral color is achieved with a C:M:Y proportion of approximately 5:4:4, so that a color approaching black can be obtained with approximately 100%C, 80%M, 80%Y. High-quality printing, however, typically requires a separate black ink.
A color can be expressed in terms of its R, G, and B values, and so can be can be thought of as being located at a point in a space, called the RGB space, where its coordinates correspond to its R, G, and B values. The RGB space is referred to as a tristimulus space since a color can be defined by three additive primaries. C, M, and Y can also be thought of as defining a tristimulus space.
There are many tristimulus spaces in addition to RGB and CMY. An international commission on illumination, known as CIE (Commission Internationale de l'Eclarage in French) defined a standard tristimulus space (known as the CIE XYZ color space) in terms of three imaginary primaries X, Y, and Z based on the human visual system.
A derived space is the xyY color space, which separates chromaticity (color-related) attributes, x and y, from a luminance-related (brightness) attribute, Y. The Yuv system, standardized in 1960, is a further derivative of XYZ. Like xyY, it has a luminance component (Y) and a two-dimensional chromaticity component (uv). Such systems are called chromaticity spaces. The Yuv system was designed to allow measurement of color differences, and exhibits a correlation between "distance" computed from coordinate differences and subjective color differences reported by observers.
FIG. 2 shows what is known as the CIE (x, y) chromaticity diagram, which contains a closed contour in x-y space. The contour includes a horseshoe-shaped line, known as the spectrum locus, with its ends joined by a straight line, called the purple line. Points on the spectrum locus represent pure colors (single wavelength), with the lefthand endpoint corresponding to 380 nm, the righthand endpoint 770 nm, and the uppermost point 520 nm. The contour encloses all human-perceptible colors. A color outside the contour has zero luminance or is indistinguishable to a human observer from a color on the contour.
Also drawn on this plot are four points, designated R.sub.meta, G.sub.meta, B.sub.meta, and D65. The first three are a particular set of additive primaries, referred to as meta RGB primaries, and D65 represents a particular white point. The meta RGB primaries (often referred to below simply as R, G, and B) are related to CIE XYZ by a predetermined (output device independent) linear transformation characterized by a non-singular (i.e., invertible) 3.times.3 transformation matrix.
A visual alternative to specifying color coordinates is to pick a color from a collection of carefully produced sample patches. The Munsell system was the first systematic approach to this kind of color specification. Munsell designed the patches to represent perceptually equal steps in each of three characteristics of color: hue (position around the color wheel), chroma (saturation, or perceived difference from a neutral color at the same brightness), and value (brightness). The Pantone system is a modern implementation of the same basic idea. Each patch in such a system has specified coordinates in the CIE XYZ system. Color patches can be thought of as a visual front end to CIE XYZ.
The CIE XYZ system classifies colors independently of the devices used to measure or reproduce them. Most GA publishing devices use numerical encodings with no clear relationship to XYZ or any other device-independent system. This means that there is no GA color exchange space. That is, there is no system of color specification that can conveniently be used with all GA publishing devices.
Desktop publishing equipment plays an increasingly large role in GA publishing. While the technological limitations of equipment for DTP are undoubtedly temporary, they pose particular problems for current GA publishing. In particular, the computational complexity of the Luv and Lab color spaces make them difficult to deal with in real time on DTP equipment.
The following publications provide additional background and are incorporated by reference: E. M. Granger, "Is CIE L*a*b Good Enough for Desktop Publishing?", Device-Independent Color Imaging, SPIE, Vol. 2170, pp. 144-148 (7-8 Feb. 1994, San Jose, Calif.); E. M. Granger, "ATD, Appearance Equivalence, and Desktop Publishing," Device-Independent Color Imaging, SPIE, Vol. 2170, pp. 163-168 (7-8 Feb. 1994, San Jose, Calif.); and E. M. Granger, "Gamut Mapping for Hard Copy Using the ATD Color Space," Device-Independent Color Imaging II, SPIE, Vol. 2414, pp. 27-35 (7-8 Feb. 1995, San Jose, Calif.).